In a compressed natural gas (CNG) fast fill process, the mass of gas contained in a receptacle cylinder is dependent on the temperature and pressure in the cylinder. It is therefore necessary for the fill system to account for variations in pressure and temperature in order to ensure that the fill process does not overfill or underfill the cylinder.
The most typical fill control system now used is an ambient temperature compensation system. In an ambient temperature compensation system, the fill control system attempts to fill the cylinder to a condition where the density of the gas in the cylinder is equal to the density of the gas at the rated cylinder pressure and the ambient temperature. However, during the fast fill process, the gas in the cylinder is compressed at a rapid rate. During this compression process, there is little time for a significant amount of heat transfer to occur, thus the gas temperature in the cylinder increases rapidly. As the fill is completed, the gas begins to cool and the pressure in the cylinder begins to decrease. As the temperature of the gas approaches equilibrium with the ambient temperature, the gas pressure in the cylinder decreases below the rated cylinder pressure. Thus, an ambient temperature compensation system results in underfilling of the cylinder because such system fails to account for the heat of compression in the cylinder arising from the fill process.
Current systems for dispensing CNG that do not use an ambient temperature compensation system likewise do not have the capability of determining and accurately compensating for the heat of compression generated in the receptacle cylinder. An example of a current dispensing system is illustrated in U.S. Pat. No. 4,527,600 to Fisher et al. In such system, the pressure and temperature at the dispenser are measured, which allows for an accurate measurement of the volume of CNG dispensed. However, like an ambient temperature compensation system, the system described in Fisher fails to account for the temperature rise in the receptacle cylinder due to the heat of compression generated during the fill process and, thus, also results in underfilling of the cylinder.
Because current systems for dispensing CNG do not have the capability of directly measuring and compensating for the temperature rise in the receptacle cylinder caused by the heat of compression inherent in the filling process, a heretofore unaddressed need exists in the industry for a system for accurately controlling the fill of CNG cylinders by monitoring and compensating for the temperature rise in the receptacle cylinder that occurs during the fill process.